Redox chemistry in thin layers of organometallic complexes prepared using ion soft landing

Abstract

Soft landing (SL) of mass-selected ions is used to transfer catalytically-active metal complexes complete with organic ligands from the gas phase onto an inert surface. This is part of an effort to prepare materials with defined active sites and thus achieve molecular design of surfaces in a highly controlled way. Solution-phase electrochemical studies have shown that V^IVO(salen) reacts in the presence of acid to form V^VO(salen)⁺ and the deoxygenated V^III(salen)⁺ complex—a key intermediate in the four electron reduction of O₂ by vanadium–salen. In this work, the V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ complexes were generated by electrospray ionization and mass-selected before being deposited onto an inert fluorinated self-assembled monolayer (FSAM) surface on gold. A time dependence study after ion deposition showed loss of O from V^VO(salen)⁺ forming V^III(salen)⁺ over a four-day period, indicating a slow interfacial reduction process. Similar results were obtained when other protonated molecules were co-deposited with V^VO(salen)⁺ on the FSAM surface. In all these experiments oxidation of the V^III(salen)⁺ product occurred upon exposure to oxygen or to air. The cyclic regeneration of V^VO(salen)⁺ upon exposure to molecular oxygen and its subsequent reduction to V^III(salen)⁺ in vacuum completes the catalytic cycle of O₂reduction by the immobilized vanadium–salen species. Moreover, our results represent the first evidence of formation of reactive organometallic complexes on substrates in the absence of solvent. Remarkably, deoxygenation of the oxo-vanadium complex, previously observed only in highly acidic non-aqueous solvents, occurs on the surface in the UHV environment using an acid which is deposited into the inert monolayer. This acid can be a protonated metal complex, e.g. [Ni^II(salen) + H]⁺, or an organic acid such as protonated diaminododecane.